1. Field of the Invention
The present invention relates to flue gas desulfurization, and more particularly to an impingement surface for the removal of wetted particles or droplets from a flue gas stream.
2. Description of the Related Art
The desulfurization of flue gas, particularly flue gas from power plants, has been the subject of considerable study. Air quality laws, both at the federal and state level, have set increasingly stringent emission standards especially for such known pollutants as sulfur oxides. The sulfur oxides, principally present as sulfur dioxide, are found in the flue gases discharged by coal and oil-fired electric power generating plants, refuse-to-energy plants, and the waste gases from other industrial processes. In addition, sulfur-containing gases, notably sulfur dioxide, may be formed in the partial combustion or gasification of sulfur containing fuels, such as coal or petroleum residuals. The control of air pollution resulting from the discharge of sulfur dioxide into the atmosphere has thus become increasingly urgent.
Sulfur oxides are produced in significant quantity by the combustion of coal or fuel oil, and a popular choice for utilities burning low sulfur fuels, for smaller industrial processes, and for refuse-to-energy plants for purposes of removing sulfur dioxide from flue gases has been the dry scrubber because of its relatively lower capital costs and its simplicity of operation. In dry scrubbing, sometimes referred to as spray drying or spray absorption, an aqueous alkaline solution or slurry is finely atomized via a mechanical, dual fluid, or rotary type atomizer and sprayed into hot flue gas to remove sulfur oxides and other contaminants. Another technical solution to reduce the emission of sulfur oxides during the production of steam for industrial uses or utility power generation is the circulating fluidized bed (CFB) reactor or combustor. In fluidized bed combustion, combustion takes place in a mixture of particles including the combustible material and limestone or dolomite which is suspended in an upwardly flowing gas stream within a furnace. The process is characterized by high heat transfer rates in the furnace, but with low combustion temperatures. The limestone undergoes a chemical reaction known as calcination within the furnace, forming calcium oxide (CaO), a sorbent which reacts with the sulfur oxides in the gas to form calcium sulfate CaSO4. CaSO4 is then removed from the system as a solid for disposal. In the CFB process, a circulating inventory of solids circulates throughout the furnace and heat transfer portions and since not all of the CaO particles react with the sulfur oxides in the gas, unreacted sorbent is carried along with the flue gas exiting from the CFB reactor.
Because of equipment space and cost limitations, the size of the dry scrubber may be such that the distance between the fluid atomizers and the absorber chamber outlet does not provide sufficient gas residence or retention time in the chamber to evaporate all of the droplets and the liquid from the wetted particles. This dry scrubber size limitation raises the likelihood of wetted particles and droplets hitting the wall of the absorber chamber and causing deposits to adhere to the internal surface of the chamber and such deposits may accumulate and interfere with the proper operation of the dry scrubber and necessitate a shutdown of the equipment to allow for a clean out of the absorber chamber internal walls. The prior art, as disclosed in U.S. Pat. No. 4,888,158 to Downs, sought to overcome this problem by introducing an inert impingement surface in the absorber chamber designed to remove only the unevaporated droplets and wetted particles from the gas stream while allowing the dry particles to pass to the particulate collection device. A shortcoming of this prior art inert impingement device is its rigid one-piece construction, which has made it difficult to remove the dried particle deposits from its surface, notwithstanding the use of rappers, sootblowers and mechanical scrappers.